\begin{landscape}
  \chapter{Planning}
  
  \begin{gantt}{7}{16}
    \begin{ganttitle}
      \titleelement{Janvier}{4}
      \titleelement{Février}{4}
      \titleelement{Mars}{4}
      \titleelement{Avril}{4}
    \end{ganttitle}
    \begin{ganttitle}
      \numtitle{1}{1}{4}{1}
      \numtitle{1}{1}{4}{1}
      \numtitle{1}{1}{4}{1}
      \numtitle{1}{1}{4}{1}
    \end{ganttitle}
    \ganttbar{task 1}{0}{3}
    \ganttbar{task 2}{2}{3}
    \ganttbar{task 3}{5}{3}
    \ganttbar{task 4}{6}{4}
    \ganttbar{task 5}{11}{4}
  \end{gantt}
\end{landscape}  
